The sensors themselves are a mixture of accelerometers, which pick up vibrations, current detectors, which measure the electrical-resistance of rocks and soil, and subsurface radar. The square kilometer they picked contains more than 100 wells, many underground storage tanks and 10s of kilometers of steel pipeline. Resead made short work of this challenge. It produced an accurate map of the area in just 10 minutes. The system could be useful for detecting tunnels on the Mexican border.
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The University of Arizona College of Engineering is testing an invisible border monitoring system that could revolutionize the way the US conducts homeland security. The border-monitoring system, known as Helios, consists of laser pulses transmitted through fiber-optic cables buried in the ground that respond to movements on the surface above. A detector at one or both ends of the cable analyzes these responses. Helios is sensitive enough to detect a dog and can discriminate between people, horses and trucks. The system can be set to avoid being triggered by small animals, and can also tell if people are running or walking, or digging, and in which direction. The location of a cut cable, or people, or vehicles, can be pinpointed instantly to within one meter along a section of cable up to 50 kilometers long.

